Today the majority of planting material is bought container grown. This can be planted at any time of the year. But from November to the end of February you can also plant bare-rooted and root-wrapped plants. These are much cheaper than their container-grown counterparts - so you can save money or buy bigger - and some nurseries offer a wider range of species and cultivars in this form.

It is possible to plant at any time during the winter providing the soil is neither frozen solid nor waterlogged. A thin crust of frozen soil won't affect planting providing it is scraped away and not placed in the planting hole.
If you can't plant bare-rooted or root-wrapped specimens immediately either heel them into a spare piece of ground to cover the roots, or put them in a frost-free shed or similar; check regularly to ensure the roots don't dry out.

Light sandy soils and heavy clay soils should be improved first with a liberal application of bulky organic matter - well-rotted manure, compost, leafmould, composted bark or whatever you can get in bulk. On heavy soils that become overly wet or even waterlogged in winter it is important to improve the planting area of at least 1 sq m/1 sq yd. If you simply dig a planting hole this will probably act as a sump and all surrounding water is likely to settle in this hole leading to root rot and subsequent plant death.

Heavy clay soils should also have the drainage improved by digging in plenty of sharp sand or grit and breaking up any compact layers - or pans - of soil.

Before planting, soak the rootball of container-grown plants and the roots of bare-rooted or root-wrapped plants in a bucket of water for half an hour to ensure they are thoroughly moist.
Always firm in the soil around the plants after planting - go easy on heavy clay soils. Water in well and for the first year after planting ensure the soil never dries out.

Containers

Nearly all winter-flowering plants can be grown in large containers. This makes them ideal for moving close to windows and doors so that you can enjoy them close by without having to venture out of doors.
When planting shrubs, trees and perennials it is important to use a compost that will sustain them for several years. Either use a straight John Innes mix or mix half-and-half with a peat or peat-substitute compost.
Even though plants won't need much watering during the winter ensure that the compost doesn't dry out at any time.

It's a good idea to plant your winter-flowering beauties close to the house so that they can be enjoyed without having to stray too far from the back door.
Many produce flowers with a gorgeous scent - another good reason to plant close to windows and doors.

Although you may think it a good idea to concentrate on planting all evergreens, it would be a shame to miss out on some of the gorgeous deciduous winter gems. Equal numbers of each will give better balance to the garden.

Aftercare

Most winter-flowering plants have a cast iron constitution and will survive the majority of weathers that winter can throw at them. However, when young they are far more susceptible.
For the first two winters it is worth keeping a careful eye on your new specimens and give protection to those you are worried about.
Draping layers of horticultural fleece over the foliage of evergreens will protect from several degrees of frost and cold winds - often responsible for more deaths than anything else.
The roots are usually the most susceptible part of any plant, so applying a thick mulch of bark chippings, straw or similar is worthwhile to provide protection.
Plants in containers are far more susceptible and it is worth taking precautions to prevent the compost freezing solid for prolonged periods:
Move them against south-facing walls
Bring them into a cold greenhouse, garage, shed or conservatory if you can.
Wrap the pots in bubble plastic or make duvets from bin liners filled with shredded newspapers, polystyrene chips, loft insulation or similar.
Cover the foliage of evergreens when strong winds are forecast.

Mix and match

'Helena contrast with the white stems of Rubus biflorus

Don't only rely on flowers for winter decoration. Many plants have colourful, evergreen foliage will set off the flowers of other plants, others provide a colourful splash with berries and seed heads, and yet others have colourful ornamental bark.

Above all enjoy your winter garden.
Established plants can have some flowering shoots pruned back and brought indoors so you can enjoy their colour and scent indoors.

I can vouch for sarcococca being ok on clay soil and the scent is lovely. Mahonia is another possibility that should be fine on clay. The M. x intermedia types flower from around Nov onwards but I think Mahonia japonica is also a winter flowering plant. If I was going to pick a viburnum for winter it would be Viburnum bodnantense 'Dawn' - lots of people have the evergreen Viburnum tinus but I find its dark green leaves a bit gloomy and heavy looking. Dawn's got a better scent anyway.